Catalyzes the radical-mediated transfer of the hydroxybenzyl group from 4-hydroxyphenylpyruvate (HPP) to 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione to form 7,8-didemethyl-8-hydroxy-5-deazariboflavin (FO).

Euryarchaeota, cyanobacteria and actinomycetes are known to produce coenzyme FO. Euryarchaeota and actinomycetes use it to synthesize coenzyme F420, whereas cyanobacteria use it for FO-dependent DNA photolyases. In actinomycetes, the two subunits cofG and cofH are fused, and are called fbiC. Possible wrong starts.